1. Field of the Invention
The invention relates to an uncoated, corrosion resistant resin delivery system that reduces thermal and shear induced degradation.
2. The Prior Art
Injection molding screws are rotatably disposed within heated barrels to process resin for injection through a nozzle. These resin delivery systems were initially formed from steel. To improve the performance, the steel base alloy was coated with various materials.
According to U.S. Pat. No. 6,634,781 the resin delivery system is formed from three layers including a base, a bond coat layer and a wear resistant coating. The base alloy may be formed from corrosion resistant alloys such as alloy steels, cast or wrought iron, low and high carbon steels, nickel based alloys and stainless steel. An intermediate layer was a coated on to the base alloy. The intermediate or bond coating layers may be made from a nickel based alloy that is welded on to the base alloy. The bond coating layer has a coefficient of thermal expansion which is about the same or less than the base alloy, and higher than the wear resistant coating. The wear resistant coating comprises a fused ceramic layer and may include materials such as chromium oxide, aluminum oxide, alumina titania, zirconium oxide, magnesium zirconate, yttria-stabilized zirconium oxide, magnesium aluminate, tungsten carbide, tungsten-chromium carbide, chromium carbide and tungsten-titanium carbide.
According to U.S. Pat. No. 6,786,630 an alternate three layer system comprises a base alloy, an intermediate layer of magnetically conductive metallic material applied by a thermal spraying process. The wear resistant layer is made up of at least 30% molybdenum or made almost entirely of molybdenum, also applied by a thermal spraying process.
U.S. Pat. No. 5,565,277 discloses a barrel comprising a bi-metallic construction, with a base metal and a coating layer consisting of microalloyed steel. The preferred micro-alloyed steels contain 0.34 to 0.40% Carbon, 0.15 to 0.80% Silicon, 1.20 to 1.65% Manganese, 0.08 to 0.13% Vanadium and the balance iron. The microalloyed steel was applied via a centrifugal casting process.
U.S. Pat. No. 7,134,868 discloses a coated nozzle having steel base material. An intermediate layer is coated with a diamond-type layer. The intermediate layer may be made from silicon, titanium or nickel formed by vacuum deposition or chemical vapor deposition. Another option would be molybdenum applied by ionization evaporation if greater adhesion is required. The diamond-type coating can be a composite type (metal and diamond mixture) or a diamond depositing type (amorphous and polycrystalline).
As will be described more fully below, all systems employing a coating have limitations. The prior art coatings were developed to improve the mechanical surface properties of resin delivery systems in a cost effective manner. However, even the most durable coatings eventually begin to break down, causing resin degradation and molding defects. These problems are exacerbated by increasing the temperature in the barrel.
Accordingly, it would be desirable to provide a resin delivery system that avoids the drawbacks of coated metals and improves the quality of defect-critical articles.